This invention relates to arc welding and more particularly to arc welding methods and apparatus in which the heat content of an alloyed wire being deposited and the associated wire feed rate are controlled independently of the welding and preheating currents.
In conventional gas-metal arc welding (GMAW) processes, heating of the alloyed wire prior to deposition is accomplished by passing welding current through a certain wire length, commonly referred to as the wire stick-out. The power consumed in heating this wire is equal to the product of the square of the welding current and the resistance of the wire (I.sup.2 R). In order to increase the wire deposition rate, the heat content of the wire is increased by increasing the electrical stick-out or the welding current or both.
Excessive wire stick-out leads to uncontrolled wire wandering and/or poor deposit quality. Therefore welding current is the predominant variable that determines deposition rate and mode of metal transfer through the arc. However, current intensification leads to more power dissipation in the arc. Since approximately 65 to 85% of the arc heat is conducted into the base metal, a higher current would increase arc penetration while increasing deposition rate and decreasing dilution. Although dilution can be reduced by employing higher welding current and slower speeds of travel, these means of control have practical limitations. Higher heat input per unit of length can generate excessive assembly distortion and metallurgical damage in both the deposit and base metal, such as heat affected zone (HAZ) underbed cracking and hot cracking.
In the conventional hot wire gas-tungsten arc welding (HWGTAW) process, heating of the alloyed wire prior to deposition is accomplished by passing a heating current through a certain length of wire stick-out. As in the GMAW process, higher wire heat content is adjusted by increasing wire stick-out or increasing heating current or both. This makes the HWGTAW process subject to similar difficulties experienced by the GMAW process. The present invention seeks to overcome these difficulties by preheating the wire remotely from the arc and molten pool, thereby reducing the required welding current and wire stick-out length.
The methods and apparatus of the present invention represent an improvement of prior art arc welding processes by providing for preheating of the alloyed wire prior to its entry into the stick-out region. In a gas-metal arc welding method where a power supply feeds welding current through a welding wire electrode into a metal workpiece, the present invention adds a preheating step whereby the wire heat content is increased by means other than the welding current. By increasing the heat content of the wire in this manner, a significant reduction in weld dilution is achieved. One means of preheating the wire is to pass current through a wire segment prior to the wire's entry into the stick-out region.
In a hot wire gas-tungsten arc welding process where an arc between a permanent tungsten electrode and the workpiece creates a pool of molten metal into which a heated welding wire is fed, again the present invention adds the step of preheating the wire. This limits the length of wire stick-out required to reach a desired wire temperature for a given current in the wire, thereby minimizing wire wandering, improving deposit quality and minimizing arc-wire interaction.
Welding apparatus in accordance with the present invention includes means for preheating welding wire such that wire heat content can be controlled independently of welding current or wire feed rate. This provides for improved control over prior art welding processes.